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Area forecast discussion 
National Weather Service Portland or
835 PM PST sun Dec 21 2014

Synopsis...an old frontal zone will linger over the Pacific northwest
through Monday as a ridgee of high pressure moves inland. The front will
shift NE some Monday night and Tuesday as a warm front...then a stronger
frontal system will bring a quick shot of precipitation Tuesday night
into Wednesday. This system will usher in a colder air mass and bring
potential for some accumulating snow to the Cascade passes later Wednesday
into Thursday. A quiet weather period appears to be in store late next
week as upper level ridging builds over the Pacific northwest. Another system
may bring bring precipitation at some point over the weekend.

&&

Short term...satellite derived precipitable water values continued
to show a relatively moist air mass remaining offshore...with
precipitable water values still just under an inch. A modest onshore
flow with westerly 800 mb winds expected to remain on the order of 20
kts overnight will continue to overrun an old baroclinic zone laid
out across the forecast area. This will continue to generate rain at
times especially over the terrain favored Cascades overnight. This evening
rainfall rates were still as high as an inch in six hours in parts
of the Oregon Cascades...but as the strength of the onshore flow
weakens overnight and Monday so too will rainfall potenitial. Another
mild night in store with dewpoints in the upper 40s and plenty of
clouds to limit radiational cooling.

Some minor tidal overflow was observed this afternoon as a high
astronomical tide combined with higher flows on the coastal
rivers. Low lying coastal areas near Willapa Bay...Astoria...Tillamook
Bay...Depoe Bay...Waldport...and Florence...along with coastal
stretches of Highway 101 near the coast will likely see some minor
issues again. With similar tidal values tomorrow afternoon...it is
expected that at least some of these locations will again see some
minor overflow issues.

Remainder of short term discussion unchanged... agreement amongst
the forecast models is increasing that there will be another period of
light precipitation late Monday into Tuesday...as a shortwave trough offshore
reactivates the stalled baroclinic zone and lifts it northward
through the region again as a weak warm front. Increased forecast probability of precipitation
during this time period...but quantitative precipitation forecast appears to fairly limited. Another
shortwave is modeled to push the boundary through as a fairly sharp
cold front on Wednesday. It appears that there will be a burst of fairly
heavy precipitation with this system...along with dropping snow
levels. The timing is somewhat uncertain at this time. The 12z European model (ecmwf)
model brings precipitation onto the coast beginning Tuesday afternoon and
spreads inland overnight into Wednesday morning. The 12z GFS and NAM are
about 6 hours slower...bringing most of the precipitation through during the
day Wednesday. This system should bring decent accumulating snowfall to
the Cascades down to the passes. Pyle

&&

Long term...no changes. Previous discussion follows. Wednesday night
through Sunday... expect lingering showers Wednesday night and Thursday behind
the departing frontal passage earlier Wednesday. Expect additional light
snowfall accumulations in the Cascades during this time. The snow
level should be below the passes...possibly even into the Cascade
foothills. Going into Thursday night and Friday...there are slight
differences between GFS and European model (ecmwf) models. The GFS develops ridging
over the Pacific northwest and keeps US dry through the rest of the week. The
European model (ecmwf) has a weaker ridge and keeps it further offshore...allowing
light showers to remain over the region into Friday. Forecast uncertainty
increases further next weekend. The GFS is dropping a low pressure
system into the interior Pacific northwest and brings a colder air mass.
Meanwhile the ewmcw is closing the low off and keeping it
offshore...which would result in a wetter and milder weekend. Will
avoid adding to much detail to the forecast during this time
period...and keep a chance of probability of precipitation through the weekend. Pyle

&&

Aviation...moderate onshore flow continues today with abundant
middle and high clouds and some rain showers remaining. VFR flight
conditions for most locations with some pockets of local
MVFR...particularly along the coast and in the south Willamette
Valley. Rain generally continues to decrease with some lowering
ceilings tonight...trending towards increasing areas of MVFR with
local areas of IFR possible overnight.

Kpdx and approaches...mostly VFR through this evening with
showers. Expect ceilings to lower a bit overnight with MVFR
conditions possible after 10z but expect VFR later Monday morning and
afternoon. /Cullen /26

&&

Marine...winds will continue to ease overnight through Monday.
Meanwhile...seas continue to run around 10 to 12 feet through much
of the week. An approaching frontal system will turn winds
southerly again late Monday or early Tuesday with the potential for Small
Craft Advisory level winds Tuesday and Wednesday and increase seas back to
around 15 feet. Seas likely finally drop below 10 feet by later Thursday or
early Friday. Cullen

&&

Pqr watches/warnings/advisories...
or...Flood Watch through Monday afternoon for Cascade foothills in
Lane County-Cascades in Lane County-central Coast Range of
western Oregon-central Oregon coast-central Willamette
Valley-Coast Range of northwest Oregon-greater Portland
metropolitan area-north Oregon coast-northern Oregon Cascade
foothills-northern Oregon Cascades-south Willamette Valley.

Coastal Flood Advisory from 10 am to 4 PM PST Monday for
central Oregon coast-north Oregon coast.

Washington...coastal Flood Advisory from 10 am to 4 PM PST Monday for
South Washington coast.

Pz...Small Craft Advisory for hazardous seas until 4 am PST Tuesday
for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.

Small Craft Advisory for rough Columbia River bar
until 10 PM PST Monday.

&&

$$

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This discussion is for northwest Oregon and southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.

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